Method and system for single ion implanation

ABSTRACT

This invention concerns a method and system for single ion doping and machining by detecting the impact, penetration and stopping of single ions in a substrate. Such detection is essential for the successful implantation of a counted number of  31 P ions into a semi-conductor substrate for construction of a Kane quantum computer. The invention particularly concerns the application of a potential across two electrodes on the surface of the substrate to create a field to separate and sweep out electron-hole pairs formed within the substrate. A detector is then used to detecting transient current in the electrodes, and so determine the arrival of a single ion in the substrate.

TECHNICAL FIELD

[0001] This invention concerns a method and system for single ion dopingand machining by detecting the impact, penetration and stopping ofsingle ions in a substrate. Such detection is essential for thesuccessful implantation of a counted number of ³¹ P ions into asemi-conductor substrate for construction of a Kane quantum computer.

[0002] An ion is an atom that has been ionised. We adopt the conventionof using the term ‘ion’ while the atom is in motion, regardless of itsionised state. After the ion has come to rest, we call it an ‘atom’.

BACKGROUND ART

[0003] The Kane computer¹ requires single donor ³¹ P atoms to be placedin an ordered 1D or 2D array in crystalline silicon. The atoms must beseparated from each other, by 20 nm or less. An alternative architectureis that of Vrijen et al.² who propose an array of ³¹ P atoms in aheterostructure where the atom spacing can be larger than the Kanecomputer but still of the order of 100 nm. Such precise positioning hasproved extremely difficult using conventional lithographic and ionimplantation techniques, or using focused deposition. This difficulty isnot only with regard to forming arrays of donor atoms with sufficientprecision, but also ensuring that only single donor atoms have beenintroduced into each cell of the array.

[0004] Optical lithography has been utilised by semiconductor industriesto manufacture integrated circuits with great precision. Opticallithography systems include an exposure tool, mask, resist andprocessing steps to accomplish pattern transfer from a mask, to aresist, and then to a device. However, the use of resist layers canlimit resolution to the wavelength of the radiation used to transfer thepattern in the mask onto the resist. This is presently about 100 nm.

[0005] Electron beam lithography, which uses a finely focused electronbeam to directly write patterns into resists, can attain better than 20nm resolution. Further, the “top-down-process”, described in a recentpatent application, uses electron beam lithography to construct arraysof nanoscale channels in resists. The resist is then irradiated with anion beam so that ions impact at random on the surface allowing a randomarray of channels to direct one or more atoms through into the substrateto construct nanoscale structures.

[0006] However, in all of these lithographic techniques, control of thenumber of atoms reaching the substrate is not possible.

[0007] Lu thi et al.⁴ describe a resistless lithography technique whichenables the fabrication of metallic wires with linewidths below 100 nm.The technique is based on an ultra-high resolution scanning shadow mask,called a nanostencil. A movable sample is exposed to a collimated atomicor molecular beam through one or more apertures in an atomic forcemicroscope (AFM) cantilever arm. Standard V-shaped Si₃N₄ cantileverswith integrated tips having a spring constant below 0.1 Nm⁻¹ were used.The aperture diameter ranged from 50 to 250 nm depending on the desiredmask structure. Scanning the sample with respect to the nanostencilallowed the structure to be laid down on the surface of the sample.After nanostructuring, the structure was inspected with the AFM tip.

[0008] This former method allows precise positioning of large numbers ofatoms but not implanting and detecting single ions.

[0009] Shinada et al.⁵ have developed a single ion detection techniqueusing a single ion implantation assembly developed by Koh et al.⁶ Thesingle ion implantation assembly consisted of a pair of deflectorplates, an objective slit, a precision quadropole-magnet, a target, anelectron multiplier tube (EMT) and a chopper control circuit connectedto the deflector plates and the EMT. The ion beam is chopped with thepair of deflector plates, over which the potential difference can beswitched. Each single ion is extracted one by one from a continuous ionbeam by adjusting the ion beam current, the objective slit diameter andthe switching time of the potential difference applied to the deflectorplates.

[0010] The extracted single ion is then focused with thequadropole-magnet lens and impacts on the target. The number of incidentions is controlled by the EMT by detecting secondary electrons emittedupon ion incidence. Signals from the EMT are fed to the chopper controlcircuit which keeps on sending the beam chopping signals to thedeflector until the desired number of single ions are detected.

[0011] Shinada et al.⁵ emphasised findings by Koh⁶ by reporting that thekey to controlling the incident ion number is the detection of secondaryelectrons emitted from a target upon ion incidence.

[0012] The secondary electron detection efficiency P_(d) is defined asfollows: ${P_{d} = \frac{N_{SE}}{N_{ext}}},$

[0013] where N_(SE) is the average number of detected secondaryelectrons by a single chop and N_(ext) is the average number ofextracted ions by a single chop, where N_(ext) is proportional to theion beam current and the time of beam chopping.

[0014] To determine the efficiency in the determination of secondaryelectrons, a 60 keV Si²⁺ ion beam was chopped with a frequency of 100kHz. NSE was estimated by dividing the number of secondary electroncounts per second by 10⁵. To evaluate N_(ext), a standard fission trackdetector was used.

[0015] The secondary electron detector included a photomultiplier tubewith a scintillator and a light guide. A grid electrode was used toguide the secondary electrons to the sensitive part of the scintillator.

[0016] The experimental result for P_(d) was 90%. The error waspartially attributed to the limitations of the secondary electrondetection system. Furthermore, results showed that the single ionincident position could be successfully controlled with an error of lessthan 300 nm.

[0017] This detection of impacts from the pulse of secondary electronsemitted from the surface due to the ion impacts does not distinguish ionimpacts with a mask from ion impacts with an exposed substrate under themask.

DISCLOSURE OF INVENTION

[0018] In first aspect the invention is a method for single ion dopingand machining by detecting the impact, penetration and stopping of asingle heavy ion in a substrate, the method comprising the steps of:

[0019] impacting electrically active substrate with single ions togenerate electron-hole pairs;

[0020] applying a potential applied across two electrodes on the surfaceof the substrate to create a field to separate and sweep outelectron-hole pairs formed within the substrate; and

[0021] detecting transient current in the electrodes and so determinethe arrival of a single ion in the substrate.

[0022] An advantage of the method is that it can be scaled to producearrays of single atoms using low energy (keV) ion implantation. Also, itis sensitive only to ions that reach the substrate and ignores ions thatstrike surface masks. It produces a record of each ion impact forverification and further analysis. The ions are detected with close to100% efficiency. And, it can be used with MeV ions to exploit the latentdamage from the passage of a single ion to nanomachine sensitivematerials.

[0023] The substrate may be a pure semiconductor substrate, such as ahigh resistivity silicon substrate. However any substrate may be usedthat is electrically active in the sense that it is ioniseable to formelectron-hole pairs with a useful lifetime.

[0024] Ions may be applied by the use of a focused beam of ions from afield ionisation ion source producing sub-20 nm ion beam probes.Alternatively, a broad-beam implanter can be used. The ion beam currentmay be adjusted to a level low enough to minimise the probability ofmultiple ion strikes during the time required to gate off the beam. Therequired current will depend on the response speed of the ion strikedetection and beam gating circuitry. Typically the current will be onehundred atoms per second. Such a beam probe can be used to inject singleions at desired locations either with or without a mask. The requiredbeam current can be tuned by using the single ion detector signalincident on a peripheral region of the substrate that is not itselfrequired in the device to be fabricated.

[0025] We will now describe the technique by which ions are detectedusing the invention. Implanted ions stop in the substrate at a depthdetermined by the initial ion energy and the stopping power of thesubstrate. There are two energy loss processes which determine thestopping power. First, nuclear processes where a close collision occursbetween a projectile and the substrate nucleus causing a recoil andstraggling. Second, electronic processes where ion kinetic energy istransferred to ionisation of the substrate and its attendant productionof electron hole pairs. It should be appreciated that only theelectronic processes produce a signal detectable by the method.

[0026] The ionisation is detected by electrodes which may be placedadjacent to the region to be implanted. Both electrodes may be on thefront surface, or one on the front surface and one on the rear surfacedepending on application. A bias voltage may be applied across them todetect the ion impacts. This leads to the possibility of measuring thepolarity of the ion-impact-induced signal as a measure of the proximityof the ion strike to the positive or negative electrode. So, it may bepossible to have two nanomachined apertures in the substrate that areimplanted with a broad beam, then the aperture which actually receivesthe ion strike could be identified from the relative strength andpolarity of the signal collected from the two electrodes.

[0027] A substrate cooling system may be required to maintain thesubstrate at a low enough temperature (of the order of 77K) to allowsufficient signal to noise ratio to detect keV ions (for MeV ions thesubstrate may be held at room temperature).

[0028] A prototype system has been shown to give very few false signals,such as random noise or from ions that do not penetrate sufficiently farinto the substrate. Pulse shape discrimination can eliminate theseevents.

[0029] Acceptable detection signals may be used to generate a gatesignal, via a computer, to a feedback circuit which may then gate offthe ion beam. Such a control signal may also step a mask to a newposition above the substrate for a further implant whereupon the beam isgated on once again.

[0030] The system may be enhanced by the use of a thin, ion sensitiveresist, that may be processed to reveal the impact sites of single ions.The incident ions pass through the thin resist and enter the substrateleaving a trail of latent damage which can be developed by standardtechniques to reveal a pit that can-be imaged with an Atomic ForceMicroscope (AFM). The resulting image of the pits reveals the siteswhere the implanted ions have entered the substrate.

[0031] The system may also be enhanced by the use of a thick resistlayer as a nanomachined mask, that blocks the ions from entering thesubstrate except for the open areas in the mask which expose the desiredareas in the substrate where single ions are to be implanted.

[0032] For the construction of a two atom device, two apertures may beopened in the mask. This may be achieved using some of the metalelectrodes in the finished device. In this case, metal electrodes arefabricated using conventional Electron Beam Lithography (EBL), then aresist layer is deposited. A cross line is drawn with the EBL systemacross the linear electrodes which upon development then opens a path tothe surface leaving the substrate exposed. The mask now consists of thethick metal electrode and the resist layer. Ions can be implanted downthe paths beside the electrodes. Some ions will stop in the metal of theelectrode, but this will not produce a signal in the ion detectionsystem because ion impacts with metals produces very little charge.

[0033] There will be an approximately 50% chance of producing a devicewith a single ion in each aperture. This chance will be actually greaterthan 50% owing to lateral ion straggling. For example, the lateralstraggling of 15 keV ³¹ P ions implanted into silicon for the Kanequantum computer is about 7 nm⁷. There is a significant probability thatthe situation where both ions entered the substrate through the sameaperture will result in the implanted atoms ending up in differentlocations. They may therefore be separately addressed with the A and Jgate electrodes of the quantum computer. There is a significantprobability that one or both of the ions will end up in the mostdesirable location under the A gate electrode itself due to ionstraggling. In any case, appropriate tuning of the gate potential canstill address the atom, even if it is not precisely located under theelectrode. Technology Computer Aided Design (TCAD) calculations showthat as long as the two atoms are in different places, they can still beindividually addressed.

[0034] The system may be used to scale up the array of implanted ions bythe use of a moveable mask consisting of a nanomachined aperture in anAFM cantilever which may be accurately positioned above the desiredlocation of the atoms and then irradiated with an ion beam.

[0035] The nanomachined apertures may be fabricated with EBL in theresist layer. Alternatively, the nanomachined aperture may be drilled ina standard cantilever and may form part of a Scanning TunnelingMicroscope (STM) or an Atomic Force Microscope (AFM). The nanomachinedaperture may be fabricated using a Focused Ion Beam (FIB) which itselfusually employs a focused beam of Ga ions, diameter less than 20 nm, toimage and machine the specimen. By first imaging the cantilever tip withthe FIB, the location of the nanomachined aperture can be thenaccurately drilled at a known location relative to the cantilever tip.

[0036] Accurate positioning of the nanomachined aperture above thespecimen may be accomplished by using the STM or AFM to first locate andimage registration marks on the substrate using the same cantilevercontaining the nanomachined aperture and to thus effectively align theaperture for an ion to pass through the aperture to implant an ion intothe substrate.

[0037] Between each implant step, the cantilever could be used to imagethe ion impact site to image chemical or morphological changes thatoccur as a result of ion impact to verify that a single ion has beensuccessfully delivered to the substrate.

[0038] The moveable mask may be controlled to a precision of less thanabout 1 nm. The thickness of the moveable mask is sufficient to stop theincident ion beam so that no ions are transmitted except through theaperture.

[0039] The system can also be used to produce scaled up arrays directlyby using a FIB to implant the ions. The focused probe in the FIB is asub-20 nm spot. In this case the focused probe is scanned over thesubstrate, dwelling on the places where the ions are to be implanted.The beam blanking and scan advance is gated on the ion impact signal.The FIB is configured to produce the ion beam required for theparticular application by use of an appropriate eutectic alloy in theion source. A combination of the nanomachined mask and the scanned FIBcan be used if the FIB probe size is larger than the apertures in themask. In this case the probe is scanned to dwell on the apertures in themask.

[0040] We will now describe a method of testing the detector. The methodmay also be used in a test mode where other ionising radiation, such asX-rays or electrons are applied to cause detectable ionisation. Such atest will confirm that the substrate is electrically active and that thesystem is working and is sufficiently efficient to detect ion impacts,before ion implantation.

[0041] This may be done with a small radioactive source (or otherappropriate source of X-rays) that is swung into place in front of thesubstrate to be implanted. The X-rays deposit the fixed amounts ofenergy, depending on the source, in the substrate without doing anydamage. A pulse height spectrum then provides an indication of thequality of the device. The X-rays penetrate surface layers and cantherefore be used even in devices that are completely covered withresist films.

[0042] A tuneable energy electron source, or a source of differentenergy x-rays, could also be used to provide multiple energy particlesfor energy calibration of the pulse height spectrum.

[0043] For all these methods, the ion-induced damage in the substratemust be annealed. After ion implantation a focused laser beam may beused to anneal the ion beam induced damage from the single ion impacts.We have shown this to work well with diamond^(8,9) where localisedregions (less then 10 microns in diameter) can be annealed withoutsignificantly heating the rest of the specimen. An alternative strategyis to use rapid thermal annealing which heats the entire substrate, butthis may cause damage to preexisting structures

[0044] In a second aspect the invention is a system for single iondoping and machining by detecting the impact, penetration and stoppingof a single ion, such as ³¹ P below 20 keV, in a substrate, comprising:

[0045] an electrically active substrate where ion or electron impactgenerates electron-hole pairs;

[0046] at least two electrodes applied to the substrate;

[0047] a potential applied across the electrodes to create a field toseparate and sweep out electron-hole pairs formed within the substrate;and

[0048] a current transient sensor to detect current in the electrodesand so determine the arrival of a single ion in the substrate.

[0049] In other applications the invention may be used to employ thepassage of a single ion to nanomachine optical fibres or other materialswith high precision. In this application the object to be machined ispositioned on top of an active substrate (which can be a commerciallyavailable particle detector). Typically MeV ions would be used whichhave a range of the order of 100 micrometers. The active substrateproduces a signal which records the passage of single ions through theobject to be machined allowing the ion beam to be stepped by one of themethods already described. After exposure in the desired locations, thelatent damage produced by the passage of single ions can be developed tocreate the nanomachined structures.

[0050] The invention may be used to control dopant implantation inintegrated chip components in order, for example, to create a regulararray of dopant atoms in the gates of transistors. Ordered arrays ofdopants may give the device desirable electrical properties for thereduction of electron scattering.

BRIEF DESCRIPTION OF DRAWINGS

[0051] An example of the invention will now be described with referenceto the accompanying drawings; in which:

[0052]FIG. 1 is a schematic diagram of an ion detection system.

[0053]FIG. 2 is a graph of an X-ray spectrum from such a system.

[0054]FIG. 3a is a graph of a pulse height spectrum of 14 keV ³¹ P ionimpacts from such a system; and FIG. 3b is a graph of a transientgenerated from one such impact.

[0055]FIG. 4 is a graph of two 14 keV ³¹ P ion impacts from such asystem.

BEST MODES FOR CARRYING OUT THE INVENTION

[0056] This example describes the invention in the context of theconstruction of a Kane quantum computer which requires ³¹ P ions with anenergy below 20 keV.

[0057] Referring first to FIG. 1 system 10 is used for detecting theimpact, penetration and stopping of a single heavy ion, such as ³¹ Pbelow 20 keV, in a substrate. The substrate 20 is a 0.2 mm thick siliconwafer of greater than 1000 Ω·cm resistivity mounted on a metal contactand earthed. The entire substrate is electrically active silicon and theimplantation of a ³¹ P ion will generate electron-hole pairs. There is alayer of oxide 5 nm thick 21 and two electrodes 22 and 23 on the surfaceof the substrate. A potential 24 is applied across the electrodes tocreate an electric field parallel with the surface to separate and sweepout electron-hole pairs formed within the substrate. A current transientsensor 30 is used to detect transient current in the electrodes and sodetermine the arrival of a single ion in the substrate. Since the device10 has no metal layer or doped layer at the surface, the dead layer 21thickness can be made much thinner than in devices constructed with ap-n junction or a Schottky structure.

[0058] The results of the charge collection efficiency measured in thesubstrate 20 improved by about 10% to at least 96% when the resistivityof the silicon substrate was increased from 1000 to around 5000-7000Ω·cm when tested with MeV ion impacts. Hence, substrates made with ahigh resistivity silicon substrate of high resistivity are most suitablein the fabrication of arrays of single ions using the detection ofelectrical transients in the substrate from ion impact method. Furtherimprovements in efficiency occur upon cooling the substrate andassociated ion detection circuitry to low temperatures, and usingSchottky barriers under the electrodes.

[0059] When an ion penetrates the substrate it excites electrons out oftheir energy levels and consequentially leaves holes. These chargecarriers are separated by an electric field applied to the electrodes.The negative charge carrier drifts towards the positive electrode andthe positive charge carriers drift towards the negative electrode with avelocity which is dependent on the electric field strength. Theresulting electrical transient is detected to generate the ion impactsignal.

[0060] If the high field region does not extend completely through thesubstrate, a dead region may exist between the electrodes correspondingto an area of low field. Any charge carriers which enter this deadregion will have a velocity close to zero and will only drift a minimaldistance and will hence recombine. Therefore the electrode configurationmust be such that the dead region is as small as possible. The movementof the remaining charge carriers constitutes a small current which canbe expressed in terms of a current transient.

[0061] The detection of a current transient, indicates that a singleatom has been implanted into the substrate at the desired location. Thesignal from the ion detection system is then used to deflect the ionbeam thereby preventing penetration of further ions.

[0062] Numerical simulations have been used to optimise the electrodepositions to maximise this signal. For ³¹ P ions with an energy up to afew 10's of keV, only about 15% of the residual kinetic energy depositedin the active layer below the oxide produces electron-hole pairs andhence a signal. The remainder, termed the pulse height defect, is lostto nuclear collisions.

[0063] Cooling the high purity substrate to the temperature of liquidnitrogen, and appropriate thermal treatment of the detector electrodesto allow large bias voltages to be applied improve system performance.

[0064] The current transient sensor 30 includes a detector preamplifierand amplifier system capable of pulse shape discrimination. Pulse shapediscrimination may be accomplished by use of a digital storageoscilloscope which digitises the entire transient caused by ion impact,or noise signal. Transient shapes which do not conform to those expectedfor ion impacts can be rejected.

[0065] The discimination can be performed by specalised electronics inthe amplifier used to produce the charge transient signal. Spectroscopyamplifiers are available commercially with in-built pulse shapediscrimination circuits (such as the ORTEC type 572) that produce areject signal when pulse pile-up is detected. Pulse pile-up is when twoion signals arrive within a short time period resulting in one pulsewith a distorted shape. Although pulse pile-up is not a problem for thestrategy outlined here, similar circuits could be used to eliminatelarge, random noise pulse on the basis of their pulse shape.

[0066] The electrical pulse height of any ion beam induced charge in thedetector system is used to register a single ion implant event. Toprevent multiple implantation of ions at the same location in thesubstrate, a fast electrostatic deflector unit located upstream of theion beam target chamber is utilised to deflect the incident ion beamafter implantation of one ion is detected.

[0067] The substrate and system are first tested by irradiating withX-rays from a radioactive source 40, for instance ⁵⁵Fe or ⁵⁷Co. TheX-rays penetrate the substrate and cause ionisation in a reversiblemanner without causing any damage. FIG. 2 is a graph of the results. Themajor peak 50 is made up of a signal peak 51 at 5.989 keV, representing⁵⁵Mn K_(α) x-rays, the decay product from ⁵⁵Fe, and a noise signal 52.There is also another minor peak centred at 6.4 keV from Mn K_(β)x-rays. The peak 50 shows the X-rays have been detected.

[0068] For ³¹P ions with an energy up to a few 10's of keV, only about15% of the residual kinetic energy deposited in the active layer belowthe oxide produces electron-hole pairs and hence a signal. Nevertheless,the noisy peak 60 shown in FIG. 3 demonstrates the the system works, andthe spectrum shows the detection of 17,000 ion impacts. The noise signal61 level of 1 keV will be reduced to below 0.5 keV with futureimprovements to the shielding of the ion detection circuits. Thecommercially available electronics for this application is rated at 0.2keV noise level which is suitable for the Kane device.

[0069] Further work has taken place with a system in which the siliconsubstrate is covered with a 60 nm resist containing two nanomachinedapertures irradiated with 15 keV ³¹ P ions. This experiment has detectedtwo single ³¹ P ions being implanted. The evidence for this is shown bythe spectra shown in FIG. 4.

[0070] In FIG. 4 the noise signal 71 is greater than before, about 3keV, so the trigger level was set at just above 3.4 keV. The experimentinvolved testing the noise signals with the beam off to set the triggerlevel above expected noise counts, and then only irradiating for a shorttime to decrease the likelihood of counting noise. A first ion ionimpact signal 80 was detected after 50 s, and another 81 after 68 s.These results were at 3.55 keV and 3.71 keV respectively and representdeeply implanted atoms that experienced greater electronic stopping andless nuclear stopping than the average. This result will be improvedlater by reduction of the noise level.

[0071] Although the invention has been described with reference to aparticular example it should be appreciated that many variations andrefinements are possible. So too are many applications for the systemand method.

[0072] Other devices will have a different configuration; the 5 nmsurface oxide described here may not necessarily be present and the beamenergy and species may be different.

[0073] The straggling caused by nuclear stopping process will introducelateral and longitudinal tolerances in the ³¹ P atom locations. Also,calculations by Koiller et al suggest that the exchange coupling betweenelectrons in silicon matrix is a strong function of separationCompensation of these effects will require appropriate potentials to beapplied to the gates associated with each qubit. These gates allow theenvironment of the qubit to be changed allowing individual qubits to beaddressed by an NMR pulse or other signals. The fidelity of thisoperation will depend on the tolerance of the qubit location and theamount of cross talk between qubits from a particular gate field. Thegate fields have been calculated by TCAD which also provides thepotential for the solutions to the Schrödinger equation allowing thequbit wave functions to be calculated. A fidelity of better than 1 partin 10⁴, required for operation of the device, can be achieved withpotentials of less than 1-2 V per electrode which is less than thebreakdown field of the oxide barrier.

[0074] An ion energy of around 15 keV is necessary to ensure the ionrange is at the required depth in the substrate which is about 20 nm forthe Kane device. A prototype quantum computer element is presently underconstruction which consists of 2 donors, to be implanted through a maskcontaining two apertures. When two ion impacts are registered, there isa 50% probability that each aperture contains 1 donor. Future deviceswill be fabricated using a focused ³¹ P beam stepped from cell to cellgated on an ion registration signal which provides the pathway toscaling up to many qubit devices.

[0075] We are also developing a moveable nanomachined mask integratedwith an AFM cantilever as another pathway to scaling up the device.

[0076] The surface of the substrate may be patterned with registrationmarks to enable the region where the single atom array is to be located.The surface may then be scanned using an AFM in order to locate theregistration marks on the surface. The known offset between thecantilever tip and a nano-machined aperture is then used to repositionthe cantilever arm with the nano-machined aperture located above thedesired location for implantation of the first atom.

[0077] The coarse positioning system may be used to move the AFM stageinto position beneath the ion beam collimator so that the ion beam canirradiate the back of the cantilever lever and illuminate thenanomachined aperture.

[0078] Using an upstream Faraday cup, the beam current from the ionsource is adjusted to a beam current of a few tens of pA. The beam isprevented from reaching the cantilever by switching on the deflectorunit. Then the beam is directed to a non-essential corner of thesubstrate to tune the beam current to a few hundred atoms per secondusing the single ion detection system.

[0079] Switching off the deflector unit allows the ion beam to irradiatethe cantilever arm.

[0080] The substrate is moved to the next location by moving the AFMstage 43. In some cases the AFM 32 can be used to image the location ofthe ion strike from the changes to the morphology of the surface causedby ion impact and hence verify the success of the ion implant. This willbe the case with MeV heavy ions.

[0081] To enhance performance charge induced in the substrate must becollected to high efficiency. The device must have a low density of freecharge carriers and a low density of defects ie., the charge carrierstrapping centres. Cooling of the substrate can be used to reduce freecarriers and also noise from the process of thermal ionisation. Withoutfree carriers a low leakage current may be sustained when a highelectrical field is applied in the sensitive volume ensuring efficientcharge separation. A low density of charge carrier trapping centres anda high charge carrier drifting velocity will reduce the loss to thetrapping centres during the charge collection. Additionally, it isdesirable that the substrate has a high breakdown electrical field, sothat high velocities of the carriers can be obtained in biased devices.

[0082] The pulse height in a device is often reduced or shows non-linearresponse to the ion energy due to three reasons:

[0083] 1. The proportion of the ions energy loss to nuclear stoppingwithout involvement in the ionisation process leading to the e-h pairsproduction (the Pulse Height Defect—PHD);

[0084] 2. Charge loss at the trapping centres during charge drift ordiffusion. This loss increases when the dense plasma produced by heavyions shields the electrical field; and

[0085] 3. Energy loss at the dead layers. Dead layers must be kept asthin as possible when keV ions are employed.

[0086] The references throughout the text above are incorporated hereinby reference:

[0087] 1. Kane, B. E., A silicon-based nuclear spin quantum computer,Nature, Vol. 393, p. 133, [1998].

[0088] 2. Vrijen, R., Yablonovitch, E., Wang, K, Jiang, H. W., Balandin,A., Roychowdhury, V., Mor, T., and DiVincenzo, C. Phys. Rev. A62 (2000)12306.

[0089] 3. PCT Application No PCT/AU01/01056 in the name of UnisearchLimited filed 24 Aug. 2001.

[0090] 4. Luthi, R., Schlittler, R. R., Brugger, J., Vettiger, P.,Welland, M. E., Gimzewski, J. K Parallel nanodevice fabrication using acombination of shadow mask and scanning probe methods. Applied PhysicsLetters, Vol. 75, Number 9, [1999].

[0091] 5. Shinada, T., Kumura, Y., Okabe, J., Matsukawa, T., Ohdormar,I. Current status of single ion implantation. Journal of Vacuum ScienceTechnologies B, Vol. 16, Number 4, [1998], pp 2489-2493.

[0092] 6. Koh, M., Igarashi, K, Sugimoto, T., Mausukawa, T., Mori, S.,Arimura. T., Ohodomori, I. Quantitative characterization of Si/Sio ₂interface traps induced by energetic ions by means of single ionmicroprobe and single ion beam induced charge imaging. Applied SurfaceScience, 117/118, [1997], pp 171-175.

[0093] 7. SRIM—The Stopping and Range of Ions in Solids, by J. F.Ziegler, J. P. Biersack and U. Littmark, Pergamon Press, New York, 1985

[0094] 8. PRAWER, S., JAMIESON, D. N. and KALISH, R.—An investigation ofcarbon near the diamond/graphite/liquid triple point. Phys. Rev. Letts69: 2991-2994 (1992).

[0095] 9. ALLEN, M. G., PRAWER, S. AND JAMIESON, D. N.—Pulsed laserannealing of P implanted diamond. Appl. Phys. Lett. 63/15: 2062-2064(1994).

What is claimed is:
 1. A method for single ion doping and machining bydetecting the impact, penetration and stopping of a single heavy ion ina substrate, the method comprising the steps of: impacting anelectrically active substrate with single ions to generate electron-holepairs; applying a potential applied across two electrodes on the surfaceof the substrate to create a field to separate and sweep outelectron-hole pairs formed within the substrate; and detecting transientcurrent in the electrodes and so determine the arrival of a single ionin the substrate.
 2. A method according to claim 1, where the substrateis a high resistivity silicon substrate and the ions are ³¹ P.
 3. Amethod according to claim 1, including the step of generating a focusedbeam of ions from a field ionisation ion source producing sub-20 nm ionbeam probes.
 4. A method according to claim 3, including the step ofgating off the beam after a single ion arrival is detected.
 5. A methodaccording to claim 1, including a preliminary step of applying ionisingradiation to cause detectable ionisation.
 6. A method according to claim5, where the ionising radiation is X-rays or electrons.
 7. A methodaccording to claim 1, including the step of measuring the polarity ofthe ion-impact-induced signal as a measure of the proximity of the ionstrike to one or other electrode.
 8. A method according to claim 1,including the step of moving a mask to a new position above thesubstrate for a further implant after a single ion arrival is detected.9. A method according to claim 1, including the steps of applying athin, ion sensitive resist to the substrate, and later processing theresist to reveal the impact sites of single ions.
 10. A method accordingto claim 1, including the steps of applying a thick resist layer to thesubstrate surface, and opening apertures in the resist for theimplantation of single ions.
 11. A method according to claim 10, wheretwo apertures are opened in the mask by electron beam lithography andsubsequent processing.
 12. A method according to claim 11, including thesteps of fabricating a linear metal electrodes on the substrate surfaceusing EBL, depositing a resist layer, drawing a cross line with the EBLsystem across the linear electrodes which upon development opens a pathto the surface leaving the substrate exposed, and implanting ions downthe paths beside the electrode.
 13. A method according to claim 8, wherethe moveable mask is a nanomachined aperture in an AFM cantilever whichis accurately positionable over the substrate surface.
 14. A methodaccording to claim 13, where the nanomachined aperture is fabricatedusing a Focused Ion Beam (FIB).
 15. A method according to claim 14,where the Focused Ion Beam (FIB) has a diameter less than 20 nm.
 16. Amethod according to claim 15, including the steps of imaging thecantilever tip with the FIB, and then drilling the nanomachined apertureat a known location relative to the cantilever tip.
 17. A methodaccording to claim 13, including the step of positioning thenanomachined aperture using STM or AFM to first locate and imageregistration marks on the substrate using the cantilever.
 18. A methodaccording to claim 13, including, between each implant step, the step ofusing the cantilever to image the ion impact site and verify that asingle ion has been successfully delivered to the substrate.
 19. Amethod according to claim 1, including the steps of dwelling a FIB on alocation on the substrate surface where an ion is to be implanted untila single ion impact is detected, and then scanning an FIB over thesubstrate to a new location, and repeating the dwelling step.
 20. Amethod according to claim 19 where the FIB is a sub-20 nm spot.
 21. Amethod according to claim 19, including the step using a nanomachinedmask and dwelling the FIB on the apertures in the mask.
 22. A methodaccording to claim 1, including the step of using a focused laser beamto anneal the ion beam induced damage from the single ion impacts.
 23. Amethod according to claim 1, including the step of cooling the substrateto allow sufficient signal to noise ratio to detect single keV ions. 24.A system for single ion doping and machining by detecting the impact,penetration and stopping of a single ion in a substrate, comprising: anelectrically active substrate where ion or electron impact generateselectron-hole pairs; at least two electrodes applied to the substrate; apotential applied across the electrodes to create a field to separateand sweep out electron-hole pairs formed within the substrate; and acurrent transient sensor to detect current in the electrodes and sodetermine the arrival of a single ion in the substrate.
 25. A systemaccording to claim 24, where the substrate is a high resistivity siliconsubstrate and the ions are ³¹ P.
 26. A system according to claim 24,including a gating subsystem to gate off the beam after a single ionarrival is detected.
 27. A system according to claim 24, includingsource ionising radiation moveable between a first position adjacent thesubstrate to cause detectable ionisation, and a second position where itdoes not irradiate the substrate.
 28. A system according to claim 27,where the ionising radiation is X-rays or electrons.
 29. A systemaccording to claim 24, including a mask moveable over the substrate toimplant a single ion in different locations.
 30. A system according toclaim 24, including a mask having two apertures.
 31. A system accordingto claim 29, where the mask is a nanomachined aperture in an AFMcantilever which is accurately positionable over the substrate surface.32. A system according to claim 31, where the nanomachined aperture isfabricated using a Focussed Ion Beam (FIB).
 33. A system according toclaim 31, where the Focused Ion Beam (FIB) has a beam of diameter lessthan 20 nm.
 34. A system according to claim 1, including a coolingsystem to cool the substrate to allow sufficient signal to noise ratioto detect single keV ions.
 35. A quantum computer fabricated using themethod of any one of claims 1 to
 23. 36. A nanomachined optical fibrefabricated using the method of any one of claims 1 to
 23. 37. Anintegrated chip having controlled dopant implantation fabricated usingthe method of any one of claims 1 to
 23. 38. A resist structure havingcontrolled dopant implantation fabricated using the method of any one ofclaims 1 to 23.